Книга: Embedded Linux Primer: A Practical, Real-World Approach

7.5.2. GRUB

7.5.2. GRUB

Many current commercial Linux distributions now ship with the GRUB bootloader. GRUB, or GRand Unified Bootloader, is a GNU project. It has many enhanced features not found in Lilo. The biggest difference between GRUB and Lilo is GRUB's capability to understand file systems and kernel image formats. Furthermore, GRUB can read and modify its configuration at boot time. GRUB also supports booting across a network, which can be a tremendous asset in an embedded environment. GRUB offers a command line interface at boot time to modify the boot configuration.

Like Lilo, GRUB is driven by a configuration file. Unlike Lilo's static configuration however, the GRUB bootloader reads this configuration at boot time. This means that the configured behavior can be modified at boot time for different system configurations.

Listing 7-11 is an example GRUB configuration file. This is the configuration file from the PC on which this manuscript is being written. The GRUB configuration file is called grub.conf and is usually placed in a small partition dedicated to storing boot images. On the machine from which this example is taken, that directory is called /boot.

Listing 7-11. Example GRUB Configuration File: grub.conf

default=0
timeout=3
splashimage=(hd0,1)/grub/splash.xpm.gz
title Fedora Core 2 (2.6.9)
         root (hd0,1)
         kernel /bzImage-2.6.9 ro root=LABEL=/ rhgb proto=imps quiet
         initrd /initrd-2.6.9.img
title Fedora Core (2.6.5-1.358)
         root (hd0,1)
         kernel /vmlinuz-2.6.5-1.358 ro root=LABEL=/ rhgb quiet
title That Other OS
         rootnoverify (hd0,0)
         chainloader +1

GRUB first presents the user with a list of images that are available to boot. The title entries from Listing 7-11 are the image names presented to the user. The default tag specifies which image to boot if no keys have been pressed in the timeout period, which is 3 seconds in this example. Images are counted starting from zero.

Unlike Lilo, GRUB can actually read a file system on a given partition to load an image from. The root tag specifies the root partition from which all filenames in the grub.conf configuration file are rooted. In this example configuration, the root is partition number 1 on the first hard disk drive, specified as root(hd0,1). Partitions are numbered from zero; this is the second partition on the first hard disk.

The images are specified as filenames relative to the specified root. In Listing 7-11, the default boot image is a Linux 2.6.9 kernel with a matching initial ramdisk image called initrd-2.6.9.img. Notice that the GRUB syntax has the kernel command line parameters on the same line as the kernel file specification.

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